CBT array center speaker data - AVS Forum
Forum Jump: 
 
Thread Tools
post #1 of 27 Old 01-05-2010, 03:16 AM - Thread Starter
AVS Special Member
 
Montekay's Avatar
 
Join Date: Apr 2007
Location: Austin TX Area
Posts: 1,037
Mentioned: 0 Post(s)
Tagged: 0 Thread(s)
Quoted: 16 Post(s)
Liked: 30
The CBT is finished! I don't have it installed in the theatre yet, I'm in San Jose, CA at the moment but if all goes to plan I will be home this weekend and have it installed by early next week.

It performs pretty much exactly as the math predicts. We made measurements on axis to 70 deg off axis in 5 deg increments but I'm posting 10 deg increments to keep the graph less crowded. The lowest curve is 70 deg off axis, just count lines up from there to see the result of 60 deg off, 50... etc. By the time you get to 30 deg off axis, the lines are very nearly aligned with the on axis response. As you can see, the off axis response drops off very uniform within the CBT's range. Starting about 240 Hz, it begins to loose control of directivity and by about 160 Hz, it's no longer acting like a CBT. Because it's open baffle however, it still maintains some control over the low frequency off axis response.

We also made an on axis measurement with and without a rear wall reflection. To do this, we set up a 4 x 8 sheet of MDF behind it at the position where the rear wall will be in the theatre. In this graph the two red curves are with the wall in place, the blue curve is without the wall. The out of control wiggly red line is with the wall bare. The other red line is with the rear wall reflection absorbed by Owens Corning 703 fiber panels. These are 2" thick. We had two layers over the entire MDF board, three layers on the bottom half of the board and then additional 2 x 4 sheets right behind the center of the speaker. We also had some 1" sheets and the thickest part was in total 15" thick. I've just ordered twenty four more 2" sheets to add to this once it's in the theatre.

We wrote all over my driveway to do this test, you can see the marks where we drew out the test points for the 5 deg increments.

We also have a 1" layer of the fiber directly behind the twenty 6" Dayton Reference drivers and the fifty Aura 2" drivers. There is no fiber behind the one hundred thirty two Tang Band tweeters because those are closed back anyway.

Given the direcivity control this speaker exhibits I think the dialog quality should be unparalleled. It's very important to absorb that rear wave however. The nice thing is that you know exactly where to put the fiber... the thing is pretty much like a magnifying glass focusing that rear wave toward the center point of the 120 deg arc of the speaker. This arc is on a 49" radius which will be the distance to the rear wall. I'm going to have a lot more absorption there than we used in the outdoor test.

Once I get it installed I'll post some photos of that. Maybe I'll eventually get around to updating my web site with more photos and details.

All seven channels of the Emotiva UPA-7 amplifier are used. Three channels for the 6" drivers and two each for the 2" and 0.5". The Behringer DCX 2496 sets the - 3 dB increments for the 2nd and 3rd driver banks on the 6" drivers. Resistors set the attenuation on the 4th and 5th banks. There is also a 0.5 dB attenuation between the crossover and the amplifier on the center bank of tweeters. It's too bad the Behringer didn't have seven outputs, that would have eliminated the need for this external 0.5 dB attenuation. It was of course measured using Arta, the mic is the Earthworks M30.

If you are not familiar with CBT (Constant Beam width Transducer) arrays google Don Keele and read his papers on the topic.

mk
LL
LL
LL
LL
LL
Montekay is offline  
Sponsored Links
Advertisement
 
post #2 of 27 Old 01-05-2010, 08:24 AM
Advanced Member
 
Rick Craig's Avatar
 
Join Date: Mar 2005
Location: Garner, NC
Posts: 508
Mentioned: 0 Post(s)
Tagged: 0 Thread(s)
Quoted: 9 Post(s)
Liked: 28
Looks cool - I need to make a trip to Austin! I was at the shop yesterday looking at some new drivers

Selah Audio

Thinking Inside the Box...
Rick Craig is offline  
post #3 of 27 Old 01-05-2010, 09:03 AM
pnw
Senior Member
 
pnw's Avatar
 
Join Date: Dec 2003
Posts: 439
Mentioned: 0 Post(s)
Tagged: 0 Thread(s)
Quoted: 7 Post(s)
Liked: 36
A truly fascinating system. Looking forward to your listening impressions!

pnw is online now  
post #4 of 27 Old 01-05-2010, 09:15 AM
AVS Special Member
 
maxmercy's Avatar
 
Join Date: Nov 2008
Posts: 2,372
Mentioned: 2 Post(s)
Tagged: 1 Thread(s)
Quoted: 42 Post(s)
Liked: 143
Montekay,

This is great stuff....it seems that if someone has a solid screen, CBT would be one of the best ways to go for center channel sound...I think I would be willing to put up with a slightly skewed center channel image to make the arc smaller than 120deg, though....but that's just me.

You have great response over a 70-80 deg arc! Amazing how deadening the front wall affects things...How did you go about designing the crossover? What did you decide on? Will it remain active, or will you build a passive version?

The vertical dispersion in one of these designs is impressive per Keele's data. Do you plan on mounting sound absorbers at ceiling reflection points?

JSS
maxmercy is offline  
post #5 of 27 Old 01-05-2010, 03:16 PM - Thread Starter
AVS Special Member
 
Montekay's Avatar
 
Join Date: Apr 2007
Location: Austin TX Area
Posts: 1,037
Mentioned: 0 Post(s)
Tagged: 0 Thread(s)
Quoted: 16 Post(s)
Liked: 30
Quote:
Originally Posted by maxmercy View Post

Montekay,

This is great stuff....it seems that if someone has a solid screen, CBT would be one of the best ways to go for center channel sound...I think I would be willing to put up with a slightly skewed center channel image to make the arc smaller than 120deg, though....but that's just me.

You have great response over a 70-80 deg arc! Amazing how deadening the front wall affects things...How did you go about designing the crossover? What did you decide on? Will it remain active, or will you build a passive version?

The vertical dispersion in one of these designs is impressive per Keele's data. Do you plan on mounting sound absorbers at ceiling reflection points?

JSS

I'm using the Behringer crossover and yes it will remain fully active. I've found the best way to go about dialing in a crossover is to first run each frequency band full bandwidth with the possible exception of some low frequency filtering on the tweeters. For example, I turn the crossovers completely off on the 6" drivers and run them flat. I then use the Behringer to EQ the response as flat a possible over as much bandwidth as possible. I notch out any peaks but never use any high Q boost. I will use shelf boost to extend the bandwidth and low Q boost to bring up any wide band low areas but never any high Q boost, if there is a narrow band notch in the response, I leave it. After that I apply the crossovers. Once this is done for all bands I level match and do any time alignment required.

I have proven to myself, forgotten or disbelieved myself and then proven to myself again that driver resonance peaks must be removed even if they are well beyond the frequency band where the driver is to be used. If you don't do this, the speaker will sound harsh. The strange thing is that this is true even if there is no visible effect to the total system frequency response measurement. That's why I always disbelieve myself and end up having to prove it to myself again.

I'm going to have a bunch of absorption behind the speaker but little if any on the side walls or ceiling. The CBT will be mounted horizontally below the screen so the CBT will control the off axis response horizontally and the dipole effect will reduce the floor/ceiling reflections. I may eventually put some absorption on the walls however. I do have one single panel on the back wall behind the back row of seats. The plan was always to put a few more but I just haven't gotten around to it. It might pay to also up a couple on the side walls down near the front just in front of the speakers.

mk
Montekay is offline  
post #6 of 27 Old 01-05-2010, 08:00 PM
AVS Special Member
 
Erich H's Avatar
 
Join Date: Dec 2007
Location: Cincinnati OH
Posts: 5,524
Mentioned: 4 Post(s)
Tagged: 1 Thread(s)
Quoted: 418 Post(s)
Liked: 711
Wow, that's some seriously incredible work. I'm in awe. That's certainly within the top 3 best builds I've seen.
Erich H is offline  
post #7 of 27 Old 01-15-2010, 07:48 PM - Thread Starter
AVS Special Member
 
Montekay's Avatar
 
Join Date: Apr 2007
Location: Austin TX Area
Posts: 1,037
Mentioned: 0 Post(s)
Tagged: 0 Thread(s)
Quoted: 16 Post(s)
Liked: 30
The CBT is up and playing in the theatre. I have to say it sounds pretty amazing! As expected dialog clarity is just outstanding no matter where you sit.

Overall, this speaker makes for an interesting listening experience. Especially when it was setup outdoors you could easily tell the directivity. You could stand in front of it and then begin walking around the arc and hear virtually no change for the first 30 deg or more each way. Then it began to drop off just slightly and once you reached the end is was almost like someone turned it off.

Another interesting is the apparent source location. It always seems the sound is coming from the center point of the arc. As you walk around it the sound follows you, it reminds me of one of those paintings where the eyes follow you as you move around the room. With it on the floor in the dining room it was facing a wall about 10 ~ 12 feet away. If you stood just off the end of the speaker you really couldn't tell anything was coming from the speaker, it sounded as if it were all coming from the wall in front of it. Because you were out of the speakers coverage area, all you could hear was the reflection off the wall. It was almost spooky how well this worked.

Setup in the theatre was pretty easy. We had dialed in the Behringer crossover to a flat response with the speaker outdoors lying flat on the ground tweeters down. Once it was mounted on the stands in the theatre it sounded thin as expected due to having been calibrated with the ground plane effect. The only correction required was to bring up the low end to where it had been with the speaker on the ground.

As usual I had to rush out of town shortly after getting it going but we did watch some scenes from the new Star Trek and later the movie, Wall E. We also moved around the room trying different seating locations and it just sounded amazing in every seat. I couldn't be happier with the performance and also with the fact that all the measurement data completely agrees with the predictions of the theory behind this speaker. I really appreciate all the support and assistance Don Keele provided on this project. At one point I can concluded it wouldn't work but Don brought me back on course and now that it's completed, I'm sold, CBT is simply amazing!

On a side note, my UPA-7 Emotiva amplifier provides the power to this thing very well. This was the final test for the Emotiva and it passed with easy. I figure this speaker has the resolution to reveal any flaw the electronics might have and I believe the Emotiva performs just as well as my Anthem. I'm selling the Anthem PVA-5 btw but I don't recommend it, you're better off saving the money and getting the Emotiva.

mk
LL
LL
Montekay is offline  
post #8 of 27 Old 11-30-2010, 11:39 PM
AVS Special Member
 
chrapladm's Avatar
 
Join Date: Mar 2010
Posts: 2,953
Mentioned: 3 Post(s)
Tagged: 0 Thread(s)
Quoted: 245 Post(s)
Liked: 101
How have you liked your CBT as of now?

I still haven't made a center channel yet and having a CBT for a center channel seems doable but I would have a much smaller one for my room. I have BFM SLA's for now for the L/R. Plus I would have to see what it would cost to build one.
chrapladm is offline  
post #9 of 27 Old 12-02-2010, 07:07 AM - Thread Starter
AVS Special Member
 
Montekay's Avatar
 
Join Date: Apr 2007
Location: Austin TX Area
Posts: 1,037
Mentioned: 0 Post(s)
Tagged: 0 Thread(s)
Quoted: 16 Post(s)
Liked: 30
Quote:
Originally Posted by chrapladm View Post
How have you liked your CBT as of now?

I still haven't made a center channel yet and having a CBT for a center channel seems doable but I would have a much smaller one for my room. I have BFM SLA's for now for the L/R. Plus I would have to see what it would cost to build one.
I've been very happy with it. Comparing it to my straight line arrays lead me to modifying the straight lines to include Hann shading. Eventually I would like to replace them with curved true CBT's. The straight lines have a "hardness" to them the CBT doesn't have. I'm not sure if simply adding the Tang Band tweeters would resolve this or not. The shading seemed to help some but it's still there. It might be due to the fact that in the straight lines the 2" Aura is used all the way to the top. I still might consider adding the tweeters leaving it straight but ultimately I think the right thing to do is to rebuild with the curve. Since these would be standing vertical, the dispersion requirement would be much less thus the arc angle would be much less. Perhaps about 30 deg as opposed to the 120 deg of the center ch. I will probably stay with a free standing CBT as opposed to ground plane just because I'm not sure about the image placement with the ground plane CBT. Inverting them and using the ceiling for the ground plane might work but I have not had the opportunity to listen to a ground plane CBT yet so I'm just not sure.

mk
Montekay is offline  
post #10 of 27 Old 12-02-2010, 07:25 AM
AVS Special Member
 
cinema mad's Avatar
 
Join Date: Jun 2006
Location: Here Nor There..
Posts: 1,839
Mentioned: 0 Post(s)
Tagged: 0 Thread(s)
Quoted: 6 Post(s)
Liked: 34
Wow MK your Center channel looks amazing ,I bet it sounds as good as it looks..

Sorry if I missed it but How long did it take to build it
how many Tweeters & mids are used & what brand..


Cheers...
cinema mad is offline  
post #11 of 27 Old 12-02-2010, 07:58 AM - Thread Starter
AVS Special Member
 
Montekay's Avatar
 
Join Date: Apr 2007
Location: Austin TX Area
Posts: 1,037
Mentioned: 0 Post(s)
Tagged: 0 Thread(s)
Quoted: 16 Post(s)
Liked: 30
Quote:
Originally Posted by cinema mad View Post

Wow MK your Center channel looks amazing ,I bet it sounds as good as it looks..

Sorry if I missed it but How long did it take to build it
how many Tweeters & mids are used & what brand..


Cheers...

Marshall cut the parts on the CNC in North Carolina then flew out to Austin for just over a week. By the time he left it was built but not fully wired. It takes a long time to wire 132 tweeters http://www.parts-express.com/pe/show...number=264-841 The 50 2" drivers are the Aura https://www.madisound.com/store/prod...oducts_id=8465 And the 20 6" drivers are the Dayton Reference http://www.parts-express.com/pe/show...number=295-342 These truncated frame Reference drivers were originally a custom driver for us but PE ordered a few extra for testing the market then made it a standard product.

mk
Montekay is offline  
post #12 of 27 Old 12-02-2010, 07:55 PM
AVS Special Member
 
chrapladm's Avatar
 
Join Date: Mar 2010
Posts: 2,953
Mentioned: 3 Post(s)
Tagged: 0 Thread(s)
Quoted: 245 Post(s)
Liked: 101
I was thinking more along the lines of a 4-5" driver and tweeter combo that can be crossed at roughly 100hz. I am mainly wondering about what size arc would I need for a seating area 9-11ft away with walls only 13.5ft wide?
chrapladm is offline  
post #13 of 27 Old 01-19-2012, 01:14 PM
 
dB-Kicker's Avatar
 
Join Date: Oct 2008
Location: Stillwater
Posts: 170
Mentioned: 0 Post(s)
Tagged: 0 Thread(s)
Quoted: 0 Post(s)
Liked: 12
Monte,

Your love of audio, and devotion to rolling your own is a boon to us all. I’ve been an avid DIY audio geek (now professional – but I still roll my own and geek out at home regularly) for ~~ 3 decades now. Having heard the dynamics and low distortion available from Line Array’s – I’ve always wanted to try them in a home theater environment.

My issue had always been two things. 1 – I’m looking for constant directivity over as large a frequency spectrum as I can attain (my holy grail). 2 - the inability to also use a line array for the center channel on its side.

I require my front three speakers to be as close to identical as possible with the same voicing, a straight non shaded line array just doesn’t have the lateral coverage necessary, lobes and combe filters too much – and won’t center the image on the screen’s center..

Your CBT Center is my answer!! I am so glad Don convinced you to give it a try! Thank you for the incredible level of work you’ve gone through in documenting it for us (well that and the actual build . I am extremely excited to start heading down the CBT path myself… Eventually I would like to build a pair of G.P. CBT’s for L/R duty, several smaller variants for surrounds, and a full CBT for the center..

I am especially happy to hear the legendre shading keeps vocals centered roughly in the middle of the screen as you move off axis. This has been my biggest sticking point – the desire to use a solid screen and maintain a fixed center image directly in the center of the screen.

On a side note – regarding your comment about your reticence to try G.P. CBT’s verses full height CBT’s above for L/R duty – you may have the answer to your fears sitting in your HT already:

After careful review of the measurements Don posted for CBT’s and ground plane CBT’s – I suspect that a half height ground plane CBT would be Ideal as replacements to your current straight line arrays..

Since you’ve already got the straight line array enclosures built repleat with HANN shading– It might be useful to digitally delay the signals and see how they sound. IOW, use digital delay and legandre shading to generate the equivalent of a full CBT, as described in don’s papers.

This way you will have a strong idea of the potential improvement in SQ going from straight to CBT array for your L/R’s.. You could also test whether you would prefer full height CBT’s or ground plane CBT’s all on the same straight L/R line arrays you currently run before ever cutting wood..

After calculating all the potential listening heights angles (which I suspect you already know  ) – choose a ground plane CBT curvature and height that properly covers the entire intended listening area, and truncate the line electronically. Only use those speakers needed. You could use delay and shading to approximate different curvatures even.. You could then invert this effect and evaluate a ceiling verses floor mounted G.P. CBT .

If you do entertain the idea of testing this – once you build what you’ve decided on, to address the harshness issue of your current L/R’s in the top octave, I recommend incorporating the tweeters used in your center for your new L/R CBT’s or G.P. CBT’s.... The majority of the answer lies in your post #5 under this thread:

Quote:
Originally Posted by Montekay View Post

I have proven to myself, forgotten or disbelieved myself and then proven to myself again that driver resonance peaks must be removed even if they are well beyond the frequency band where the driver is to be used. If you don't do this, the speaker will sound harsh. The strange thing is that this is true even if there is no visible effect to the total system frequency response measurement. That's why I always disbelieve myself and end up having to prove it to myself again.

Those Aura’s have a breakup mode above 10K that can be heard, and I suspect this is the majority of the harshness you currently experience with your L/R’s relative to your center even after shading..

I gotta say – at some point I would love to head down to Austin and give your system a listen!!
dB-Kicker is offline  
post #14 of 27 Old 01-19-2012, 02:39 PM
AVS Special Member
 
maxmercy's Avatar
 
Join Date: Nov 2008
Posts: 2,372
Mentioned: 2 Post(s)
Tagged: 1 Thread(s)
Quoted: 42 Post(s)
Liked: 143
dB,

When you build yours, we need a thread....

I promise to do the same when I build some as well...

A ceiling mounted groundplane CBT is near-ideal as a surround, with absorption behind it to avoid 1/4 wave cancellations.

Only thing a center channel CBT needs is absorption behind it and at first reflection points floor and ceiling.

I have said many times that CBT is a great option for those without acoustically transparent screens....

JSS
maxmercy is offline  
post #15 of 27 Old 01-19-2012, 02:55 PM
AVS Special Member
 
Bigus's Avatar
 
Join Date: Feb 2003
Location: The South
Posts: 4,258
Mentioned: 0 Post(s)
Tagged: 0 Thread(s)
Quoted: 0 Post(s)
Liked: 51
dB, why must you use a horizontal orientation for a center channel line array? Not familiar with your system... no option for an acoustically transparent screen I suppose?

Looks like the CBT would be ideal if you have to use a horizontal center.

Bigus is offline  
post #16 of 27 Old 01-19-2012, 04:14 PM
 
dB-Kicker's Avatar
 
Join Date: Oct 2008
Location: Stillwater
Posts: 170
Mentioned: 0 Post(s)
Tagged: 0 Thread(s)
Quoted: 0 Post(s)
Liked: 12
Quote:
Originally Posted by maxmercy View Post

dB,

When you build yours, we need a thread....

I promise to do the same when I build some as well...

A ceiling mounted groundplane CBT is near-ideal as a surround, with absorption behind it to avoid 1/4 wave cancellations.

Only thing a center channel CBT needs is absorption behind it and at first reflection points floor and ceiling.

I have said many times that CBT is a great option for those without acoustically transparent screens....

JSS

Absolutely! Just brainstorming at the moment, but I'm thinking about using the same horns and mids my system currently uses. A Left and Right cutaway view of the current take on a L/R GP CBT using them is attached along with the current settup.

Quote:
Originally Posted by Bigus View Post

dB, why must you use a horizontal orientation for a center channel line array? Not familiar with your system... no option for an acoustically transparent screen I suppose?

Looks like the CBT would be ideal if you have to use a horizontal center.


My HT also doubles as the familly room. To help with some of the lighting issue's - I use a hard screen with high gain. (Vutech Silver Star)
LL
LL
dB-Kicker is offline  
post #17 of 27 Old 01-19-2012, 06:14 PM
AVS Special Member
 
Brad Horstkotte's Avatar
 
Join Date: Dec 2005
Location: Torrance, CA
Posts: 5,123
Mentioned: 0 Post(s)
Tagged: 0 Thread(s)
Quoted: 12 Post(s)
Liked: 57
Fascinating - routing / sanding / wiring those tweeters must have been FUUUUUNNNNN. I just had to do it for a pair of baffles with 16 tweeters each, and that was already tedious.
Brad Horstkotte is offline  
post #18 of 27 Old 01-19-2012, 07:56 PM
Member
 
Viperoni's Avatar
 
Join Date: Jun 2011
Posts: 95
Mentioned: 0 Post(s)
Tagged: 0 Thread(s)
Quoted: 0 Post(s)
Liked: 11
WOW.

I feel the need to point out that with just the 20x 6" drivers, you have in the range of 2000 sqcm of surface area, more than 2 15's.
Viperoni is offline  
post #19 of 27 Old 01-19-2012, 08:29 PM
AVS Addicted Member
 
LTD02's Avatar
 
Join Date: Oct 2003
Posts: 16,840
Mentioned: 23 Post(s)
Tagged: 0 Thread(s)
Quoted: 554 Post(s)
Liked: 1163
"Marshall cut the parts on the CNC in North Carolina then flew out to Austin for just over a week."

sounds pricey...;-)

though i like what you guys have there...

Listen. It's All Good.
LTD02 is offline  
post #20 of 27 Old 01-19-2012, 08:37 PM
AVS Special Member
 
maxmercy's Avatar
 
Join Date: Nov 2008
Posts: 2,372
Mentioned: 2 Post(s)
Tagged: 1 Thread(s)
Quoted: 42 Post(s)
Liked: 143
Quote:
Originally Posted by dB-Kicker View Post


Absolutely! Just brainstorming at the moment, but I'm thinking about using the same horns and mids my system currently uses. A Left and Right cutaway view of the current take on a L/R GP CBT using them is attached along with the current settup.

My HT also doubles as the familly room. To help with some of the lighting issue's - I use a hard screen with high gain. (Vutech Silver Star)

I also do not have full light control and use a gray screen, nor the space for a proper baffle wall and AT screen....

The limitation I see will be w/ the horns. C-C spacing will not be ideal, but I don't know if anyone has measured a CBT w/ horns for high end. Their directivity may be a good thing, not studied in a CBT orientation, AFAIK.....

JSS
maxmercy is offline  
post #21 of 27 Old 01-20-2012, 06:40 AM
Advanced Member
 
Rick Craig's Avatar
 
Join Date: Mar 2005
Location: Garner, NC
Posts: 508
Mentioned: 0 Post(s)
Tagged: 0 Thread(s)
Quoted: 9 Post(s)
Liked: 28
Quote:
Originally Posted by maxmercy View Post

I also do not have full light control and use a gray screen, nor the space for a proper baffle wall and AT screen....

The limitation I see will be w/ the horns. C-C spacing will not be ideal, but I don't know if anyone has measured a CBT w/ horns for high end. Their directivity may be a good thing, not studied in a CBT orientation, AFAIK.....

JSS

I've measured the arrays with both ribbons and domes but not horns. I would never use a horn in a CBT due to the driver spacing. The measured results are very predictable in terms of where the tweeters start to go into the "chaos" region.

Selah Audio

Thinking Inside the Box...
Rick Craig is offline  
post #22 of 27 Old 01-20-2012, 05:24 PM
 
dB-Kicker's Avatar
 
Join Date: Oct 2008
Location: Stillwater
Posts: 170
Mentioned: 0 Post(s)
Tagged: 0 Thread(s)
Quoted: 0 Post(s)
Liked: 12
Rick - Max, emailed you off line, don't want to clutter Monte's CBT Center thread up..
dB-Kicker is offline  
post #23 of 27 Old 01-20-2012, 06:43 PM
AVS Addicted Member
 
noah katz's Avatar
 
Join Date: Apr 1999
Location: Mountain View, CA USA
Posts: 20,732
Mentioned: 1 Post(s)
Tagged: 0 Thread(s)
Quoted: 367 Post(s)
Liked: 180
Very impressive design/build, and horizontal directivity performance.

Quote:
Originally Posted by maxmercy View Post

The vertical dispersion in one of these designs is impressive per Keele's data.

Not to be a wet blanket, but if the goal is directivity control, shouldn't that be *de*pressive?

Isn't there very little vertical directivity control?

Noah
noah katz is online now  
post #24 of 27 Old 01-20-2012, 07:03 PM
AVS Special Member
 
maxmercy's Avatar
 
Join Date: Nov 2008
Posts: 2,372
Mentioned: 2 Post(s)
Tagged: 1 Thread(s)
Quoted: 42 Post(s)
Liked: 143
'uninhibited' would have been a better term. Absorption to avoid ceiling/floor reflecions in a horiz. CBT would be good. But, no different than with non-directivity controlled speakers....

JSS
maxmercy is offline  
post #25 of 27 Old 01-21-2012, 09:01 AM - Thread Starter
AVS Special Member
 
Montekay's Avatar
 
Join Date: Apr 2007
Location: Austin TX Area
Posts: 1,037
Mentioned: 0 Post(s)
Tagged: 0 Thread(s)
Quoted: 16 Post(s)
Liked: 30
Quote:
Originally Posted by dB-Kicker View Post


Since you've already got the straight line array enclosures built repleat with HANN shading- It might be useful to digitally delay the signals and see how they sound. IOW, use digital delay and legandre shading to generate the equivalent of a full CBT, as described in don's papers.

That would be cool however, the drivers are wired in groups. The delay, unlike the shading can not be done in groups, it would need to be done by individual drivers. So in order to do digital delay simulations the lines would need to be rewired with individual lines going to each flanking pair starting from the center. For the 6" driver this would require 9 digital delays and for the 2" drivers it would require 21 individual digital delays. It would be cool to have a straight line set up this way to experiment with but unfortunately it would not be practical to use it to determine the optimum redesign of the current mains. I know this because when working on the center speaker design I suggested to Don that the speaker be built straight and use delays on each group of drivers and he shot me down.

mk
Montekay is offline  
post #26 of 27 Old 01-21-2012, 11:11 PM
 
dB-Kicker's Avatar
 
Join Date: Oct 2008
Location: Stillwater
Posts: 170
Mentioned: 0 Post(s)
Tagged: 0 Thread(s)
Quoted: 0 Post(s)
Liked: 12
Quote:
Originally Posted by Montekay View Post

It would be cool to have a straight line set up this way to experiment with but unfortunately it would not be practical to use it to determine the optimum redesign of the current mains.mk

Monte,

Bummer! I wonder if it would be possible for you to borrow a couple smaller CBT's, say a pair's of CBT 36's or similar to test with. Stack two together one on top of an upside down variant for the full CBT test, then just the GP variant. Might even be able to hang it to check ceiling vs floor coverage.

Sure would be nice to know before you go through the effort of rebuilding your mains!!!

Quote:
Originally Posted by noah katz View Post

Isn't there very little vertical directivity control?

Noah,

The arc in degree's combined with the legendre shading do give vertical controll. As an example - with roughly 60dg arc (and 12 drivers), you have about 45dg of vertical spread with very even coverage, then a dramatically reduced level as you increase height - but with very similar F/R to the area in coverage.

Keel show's it pretty well in this paper:

B&W 801 VS CBT36 measurements.http://audioartistry.com/brochures/B...nts%20v8.1.pdf

Response vs height measurements begin on page 12, and run through 16, ceiling coverage starts on 17 running to page 21.
dB-Kicker is offline  
post #27 of 27 Old 01-22-2012, 09:16 AM
AVS Special Member
 
DS-21's Avatar
 
Join Date: Jun 2005
Posts: 3,553
Mentioned: 0 Post(s)
Tagged: 0 Thread(s)
Quoted: 209 Post(s)
Liked: 245
Quote:
Originally Posted by dB-Kicker View Post

The arc in degree's combined with the legendre shading do give vertical controll. As an example - with roughly 60dg arc (and 12 drivers), you have about 45dg of vertical spread with very even coverage, then a dramatically reduced level as you increase height - but with very similar F/R to the area in coverage.

Keel show's it pretty well in this paper:

B&W 801 VS CBT36 measurements.http://audioartistry.com/brochures/B...nts%20v8.1.pdf

But for a horizontally-arrayed CBT, one needs to look at Dr. Keele's horizontal measurements to determine vertical coverage. The CBT doesn't look that good in the midrange in that respect. (See pg. 31-2.)

That said, I've also seen smart people make the argument that holes in the power response are relatively benign. (If memory serves, Dr. Keele may have been one of them.) The CBT design Dr. Keele designed and measured doesn't seem to have any off-axis "horns," just holes.

--
"In many cases there aren’t two sides unless one side is 'reality' and the other is 'nonsense.'" - Phil Plait
Serious Audio Blog 
Multichannel music (and video) urban loft living room system 
DS-21 is offline  
Reply DIY Speakers and Subs

Thread Tools
Show Printable Version Show Printable Version
Email this Page Email this Page


Forum Jump: 

Posting Rules  
You may not post new threads
You may not post replies
You may not post attachments
You may not edit your posts

BB code is On
Smilies are On
[IMG] code is On
HTML code is Off
Trackbacks are Off
Pingbacks are Off
Refbacks are Off